The present invention relates to a female snap button, and especially to a female snap button made of synthetic resin.
A snap button, which is widely used for parts to be put together of clothing etc., consists of a male snap button (male snap) as fixed to one of the clothing parts and a female snap button (female snap) fixed to the other. The male and female snaps are connected and disconnected with each other by mating and demating a projection of the male snap with and from a projection-receiving space of the female snap. A female snap made of synthetic resin can obtain flexibility (elasticity) more easily than a metal snap for mating and demating the projection with and from the projection-receiving space. A female snap as injection-molded of thermoplastic resin is disclosed in e.g. Japanese Examined Utility Model Application Publication No. H07-3924, and comprises generally a disk-like base and a cylindrical, annular protrusion rising from the base defining a projection-receiving space on the base. At a distal end of the inner peripheral surface of the annular protrusion, there is formed an inner bulge bulging radially inward. On the other hand, at a distal end of the projection of the male snap button, there is formed an outer bulge bulging radially outward. When the projection of the male snap is put in and taken out from the projection-receiving space of the female snap, the outer bulge of the projection first overlaps the inner bulge of the annular protrusion, which elastically displaces the annular protrusion radially outward. Then, the outer bulge of the projection passes the inner bulge of the annular protrusion, and thereupon the annular protrusion is radially restored. Thereby, connecting or disconnecting the male snap with the female snap is completed. In the state with the male and female snaps connected, the outer bulge of the projection catches on the inner bulge of the annular protrusion, which prevents the projection from being easily removed from the projection-receiving space of the female snap.
In a conventional synthetic-resin female snap, the inner and outer peripheral surfaces of the annular protrusion are in a horizontally perfect circle shape with a constant radial thickness of the annular protrusion in the whole circumferential direction. Therefore, when the projection of a male snap is put in and taken out from the projection-receiving space of the female snap, the flexibility of the annular protrusion as being elastically deformed radially outward is uniform in the whole circumferential direction. For this reason, very high precision is required for the flexibility of the annular protrusion, so it is not easy to produce female snaps. Further, to enhance a resistance in the projection-receiving space to detachment from the projection, there is a problem that a force required to mate and demate the projection with and from the projection-receiving space will be increased. In addition, because of faulty molding or long-term use, there would be nonuniformity in the detachment resistance or the mating and demating force in the circumferential direction such that the projection would be easily removed from the projection-receiving space at a certain point in the circumferential direction.    [Patent document 1] Japanese Examined Utility Model Application Publication No. H07-3924
An object of the invention as made in view of problems as mentioned above is to provide a female snap button which does not require high precision in terms of the flexibility of the annular protrusion, can reduce a force to mate and demate the projection with and from the projection-receiving space while maintaining a relatively high resistance in the projection-receiving space to detachment from the projection, and it is unlikely to bring about nonuniformity in the detachment resistance or the mating and demating force in the circumferential direction.